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I'm a new home brewer. Since Christmas I've brewed 5 beers, all of which have been drinkable. The first I didn't have a hydrometer, the second I accidentally put in twice as much carapils as I was supposed to, but the last three have all given me the same anomaly in that og and fg are much lower than stated in the recipe. I can understand og being lower, and am aware of causes of mash inefficiency, some of which I intend to try and mitigate in future brews. What I don't understand is why the og and fg are off by the same amount.

For example my last brew is a Belgian strong golden ale. OG in the recipe was 1.070, and fg was 1.012. my gravities were 1.060 and 1.003. I would expect, if the amount of fermentables was in proportion, that an OG of 1.060 would give a FG more like 1.010. If for some reason the amount of fermentables was not in proportion (i.e the mash resulted in a lower OG of mostly fermentable sugars) I would expect a watery tasting beer, which they're not.

Has anyone experienced anything similar or have any ideas what is happening?

I've used two different hydrometers and have tried to degass samples post fermentation so I don't think these are factors. I'm doing a single step infusion mash in a cooler, which sits at 65c for an hour and maybe loses 1c during that time, followed by 2x batch sparges involving a good stir and 10 mins sitting in ~65c water, then vorlaufing again before running off.

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Your mash temp favors beta-amylase which makes a more fermentable wort.

While beta-amylase denatures beginning at 149°F / 65°C it takes a little time.

It's possible your thermometers are slightly out of adjustment and you may be further in beta range than realised.

For a less fermentable wort mash at 154-156°F, this will denature beta-amylase quicker and allow alpha-amylase to convert your starches.

As far as a dry beer 1.003 FG, it won't always be watery or even light in mouth feel if there are proteins to augment it.

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    Thanks, I am mashing at 67 as we speak and using a bit more grain to compensate for any lost fermentables / general efficiency. I'm also testing my pH but only have capsules and vial testing kit to hand, which currently appears "beige".
    – David Liam Clayton
    May 16, 2018 at 13:48
  • @DavidLiamClayton do an iodine test to know when your mash is all converted.
    – Evil Zymurgist
    May 16, 2018 at 15:31
  • iodine testing kit is somewhere on my list of bits and bobs to buy, along with litmus paper. After I got back from taking daughter to the park my ph sample had cleared, and looked like a 6.
    – David Liam Clayton
    May 16, 2018 at 17:10
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    @DavidLiamClayton don't need an iodine "kit" DYI your own, clean eye dropper, white glass plate or juice bottle lid, iodine from pharmacy. Few drops of grain free wort on the white surface, one drop iodine. When it goes mostly yellow mash is complete, any black to deep orange still has starches.
    – Evil Zymurgist
    May 18, 2018 at 12:58
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My first thought would be to check your efficiencies, if the recipe targets 1.070, usually aimed around 75% efficiency, and you only reach 1.060 (=56%?) than your mash is off. Being a Belgian strong I guess this beer used some sort of sugar adjunct, so keep that in mind.

But if your hydrometers are off the measurements are wrong of course. Perhaps ‘calibrate’ it with a sugar solution? Dilute a specific amount of sugar in a specific amount of water to get a pre calculated gravity.

The mash temp is fine, heavy Belgians usually tend to aim for a very fermentable wort. How often do you check the temp during the mash, and do you stir it? I’ve heard other brewers have similar results, due to loosing too much heat on the sides of the mash tun and thus parts of the mash basically being too cold. Could be the issue?

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  • Thanks, yes I understand some of the common issues, just wasn't sure why it attenuates by the expected gravity when OG is so much lower than expected. The recipe included candi sugar, but that went in the boil and I measured OG post-boil (assume this is right) so that should be factored in already. I don't stir the mash, but give it a good mix to begin so there are no dry spots, and then take the temp periodically. Having said this, it's possible last brew I got different temps for the top and bottom of the mash, so perhaps stirring it would help to ensure the same temp throughout.
    – David Liam Clayton
    May 16, 2018 at 11:01
  • also, can you elaborate on what you mean by "your mash is off"?
    – David Liam Clayton
    May 16, 2018 at 17:11
  • By an ‘off mash’ I mean the usual causes of a lower efficiently, like a too coarse mill size or dough balls. But like you said you are aware of those, and it is also not the actual question. Going from 1.060 to 1.003 is a very (very) high attenuation level but not impossible due to the recipe and yeast (mostly base grains, adjunct sugars and the Belgian yeast strains which can be quite aggressive).
    – JesseB1234
    May 17, 2018 at 9:08
  • About the not-watery taste. Like Evil Zymurgist said, a low FG doesn’t have to mean a watery taste. For example, the flagship beer of this style, a Duvel has an estimated FG of 1.004. With an abv. of 8.5% this means a OG of about ~1.070. The low FG balances out the high alcohol and together with a high carbonation and the yeast esters/phenols give a perception of a refreshing beer (while actually knocking you of your feet, hence the reference to the devil).
    – JesseB1234
    May 17, 2018 at 9:09
  • I see, thanks for the explanation. I buy pre-milled grain online, so that's effectively out of my control. From yesterday it appears that my water is probably a bit on the alkaline side. I guess trying to get mash ph right and maybe iodine testing for conversion would be sensible low-cost places to start. Thanks again. And if it's anything remotely like Duvel I'll be chuffed.
    – David Liam Clayton
    May 17, 2018 at 13:48

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